Technology of conventional example 1 provides an arc welder formed of a combination of a robot controller of a teaching playback type robot and a control portion of a welding power source as disclosed in Japanese Patent Unexamined Publication No. H9-85443. The arc welder is controlled in the following steps of:                transmitting a welding current command signal and a welding voltage command signal as analog voltage signals to the robot controller;        converting each inputted analog command signal to digital data with the welding power source; then        determining a welding current command value and a welding voltage command value; and        capturing them with a welding condition control portion.        
Technology of conventional example 2 provides a welding apparatus formed of a combination of a robot controller body of a teaching playback type robot and a control portion of a welding power source in FIG. 16. This technology is disclosed in Japanese Patent Unexamined Publication No. 2000-117435. Both the robot controller body and the control portion perform digital control, and have a digital communication control portion. A welding condition command including a welding current command value is digitally transmitted from the robot controller body to the control portion of the welding power source through the communication control portions.
Technology of conventional example 3 is disclosed in Japanese Patent Unexamined Publication No. H10-58157, and employs system bus 221 as shown in FIG. 17. In conventional example 3, welding current controller 228 controls welding current flowing through an electrode for spot welding (not shown). Robot central processing unit (CPU) portion 220 controls synchronously welding current controller 228 and applied pressure controller 227 via system bus 221.
Robot CPU 220 controls the controllers based on welding condition data stored in a memory so that applied pressure and welding current change synchronously in a plurality of stages.
Technology of conventional example 4 employs an arc welding sensor as shown in FIG. 18. Communication by the arc welding sensor and communication of a control portion of a welding condition in the welding power source are independent.
In conventional example 1, the control systems are digital circuits including a microcomputer as a main body. In the interface with the welding power source as discussed above, the welding control is performed in the following steps of:                digital-analog (D/A) converting the digital welding current command value and welding voltage command value to analog command values on the robot side based on respective output characteristic curves;        transmitting the analog values to the welding power source side; and        analog-digital (A/D) converting the analog values to digital values again on the welding power source side based on respective output characteristic curves.        
Analog circuits are thus interposed though the control systems are the digital circuits, so that a conversion error occurs. There are the following problems. The welding current command value and welding voltage command value transmitted from the robot side are not equal to the welding current command value and welding voltage command value received by the welding power source side. The control systems are affected by analog drift by environment change (especially, temperature) and secular change.
Conventional example 2 addresses the problems of the analog interface of conventional example 1. The robot controller body and the control portion of the welding power source perform digital control, and have the digital communication control portion. A welding condition command is digitally transmitted from the robot controller body to the control portion of the welding power source through the communication control portions by a serial communication method. Analog conversion period in conventional example 1 is 70 to 80 msec. While, serial communication period in conventional example 2 is 9 to 10 msec, namely smaller comparing with the conventional example 1.
In performing the welding, however, work for starting the welding often takes much time even in conventional example 2. For example, at least, a welding start command (arc-on sequence) must be executed in starting arc welding as in the sequence of FIG. 9A, and a welding end command (arc-off sequence) must be executed in finishing the arc welding as in the sequence of FIG. 9B.
A welding control portion spends much time on welding control itself, so that the welding control portion can make a welding error when much information is intended to be communicated between the robot controller body and the control portion of the welding power source during the welding control.
For preventing such a problem, the execution of a series of welding start commands or welding end commands is started slightly before a welding start point or a welding end point. In this method, the control is triggered depending on timing, so that the start of the arc welding can be failed when the timing is off.
When the robot body intends to move to a next teaching point while a wire is stuck to a work piece, the robot body can damage a torch. When a serial communication for transmitting information between a control portion of an arc sensor or the like and the robot controller is added to the welding apparatus, real time control can be damaged.
When serial communication speed is simply increased, many communication failures occur especially due to tungsten-inert-gas (TIG) high-frequency noise under welding environment, thereby resulting in delay by a re-communication process.
In conventional example 3, the applied pressure controller, the welding current controller for controlling the welding current flowing through the electrode for spot welding, and a welding condition database are interconnected through the bus. Therefore, information transmission path is shortest, and the robot CPU can perform easy, speedy, and accurate control without requiring complex timing adjustment of a communication procedure or the like.
However, the welding apparatus in conventional example 3 switches between the applied pressure control and the welding current control, simply using a welding elapsed time as a trigger in welding. The welding condition database including welding output characteristic curves is directly read from the bus of the robot CPU. The robot CPU captures welding current information from a welder, real applied pressure information by a load cell or the like, and feedback information of a chip's tip position by a laser sensor, and synchronizes the real applied pressure with the welding current in real time. Thus, optimal adaptive control is realized in response to a behavior of a work piece. However, optimal adaptive control is not realized over the whole welding route having a certain length of the arc welding.
The welding apparatus in conventional example 4 has an arc welding sensor as discussed above. The arc welding sensor includes a current detector for detecting required real welding current and a resistor for detecting required real welding voltage, and the current detector and the resistor are interconnected in a circuit. Additionally, this welding apparatus includes similar detectors for the real welding current and voltage for welding control also in the welding power source.
A bleeder resistor in the welding power source and the resistor for detecting welding voltage disposed in the arc welding sensors are interconnected in parallel between the + output terminal and the − output terminal of the welding power source, so that welding voltage drops. A DCCT (Hall device) in the welding power source and a current detector disposed in the arc welding sensors are connected to the + output terminal of the welding power source in series and the impedance increases, so that detected current value deviates from a true current value especially in steep change of welding current.
Information communication speed on a serial communication line is largely restricted depending on noise resistance and real time processing performance, so that the arc sensor cannot be precisely controlled during welding.